In the fabrication of articles such as cables, pipes, footwear, foams and the like, the polymeric compositions from which these articles are made must often be melt blended. The compositions often comprise silane-functionalized resins and a catalyst, and these resins undergo crosslinking through their silane functionalities upon exposure to moisture at either an ambient or an elevated temperature. Moisture-cured resins represent a significant portion of the market for crosslinked polyolefins in cable insulation today. They are generally restricted to articles of thin construction because the crosslinking chemistry requires the polymer to absorb moisture from the environment while below the melting point, and diffusion of water through semicrystalline, hydrophobic polymer is very slow.
Direct addition of water prior to fabrication is impractical because (a) it boils at temperatures appropriate for the extrusion of polyethylenes; (b) its solubility in polyolefins is extremely low; (c) water trapped in the polymer above the saturation point is likely to phase separate, causing defects due to voids and highly non-uniform crosslink densities. Also, large amounts of free water might cause premature crosslinking.
One key requirement in the moisture cure process is to minimize premature crosslinking of the resin during storage or during melt processing, e.g., extrusion, molding, etc.
Another important consideration in the fabrication of these articles is to achieve crosslinking within a short period of time, e.g., hours, days, after the melt processing is completed. Short cure times can be achieved by curing at elevated temperatures, e.g., in excess of 70° C., and/or through the use of powerful catalysts such as sulfonic acids. As the thickness of the fabricated article increases, so does the time that moisture takes to diffuse into and through the polymer composition, even at elevated temperatures. This adds to the cost of the fabrication process. As such, the polymer fabrication industry has a continuing interest in accelerating the moisture-cure of silane-functionalized resins.